RT4841 datasheet(12/16 Pages) RICHTEK

12 / 16 page

RT4841

DS4841-00 May 2021

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

SWO

I(SWO)

GD SCP

Timer

5mS

SWO

I(SWO)

GD SCP

Timer

5mS

12V

200mA

Without trigger

GD SCP

With trigger

GD SCP

Inductor Short Protection

The RT4841 step-up converter has diode short protection

to prevent the IC damage. It prevents huge current from

input supply and output capacitor to damage internal power

MOS. During power on, once happen inductor short and

then internal MOSFET will turn off immediately.

Inductor Selection

The inductor value depends on the maximum input current.

As a general rule the inductor ripple current is 20% to

40% of maximum input current. If 40% is selected as an

example, the inductor ripple current can be calculated

according to the following equation :

OUT

OUT(MAX)

IN(MAX)

RIPPLE

IN(MAX)

x I

x V

= 0.4 x I



where

η is the efficiency of the boost converter, IIN(MAX) is

the maximum input current, and IRIPPLE is the inductor

ripple current. The input peak current can be obtained by

adding the maximum input current with half of the inductor

ripple current as shown in the following equation :

PEAK

IN(MAX)

I1.2 x I



Note that the saturated current of inductor must be greater

than IPEAK. The inductance can eventually be determined

according to the following equation :

OUT

OUT(MAX)

OSC

x (V ) x(V

V )

0.4 x (V

) xI

x f







where fOSC is the switching frequency. For better system

performance, a shielded inductor is preferred to avoid EMI

problems.

Output Capacitor Selection

Output ripple voltage is an important index for estimating

the performance. This portion consists of two parts, one

is the product of IIN and ESR of output capacitor, another

part is formed by charging and discharging process of

output capacitor. As shown in Figure 3,

ΔVOUT1 can be

evaluated based on the ideal energy equalization.

According to the definition of Q, the Q value can be

calculated as following equation :

OUT

OUT1

OUT

OSC

x V



 



  



  

 





 







where fOSC is the switching frequency and

ΔIL is the

inductor ripple current. Move COUT to left side to estimate

the value of

ΔVOUT1 as following equation :

OUT

OUT1

OUT

OSC

D x I

x C

x f





Finally, the output ripple voltage can be determined as

following equation :

OUT

OSC

D x I

ESR

x C

x f









Time

Inductor Current

Output Current

Output Ripple

Voltage (ac)

(1-D)TS

ΔVOUT1

ΔIL

Input Current

Figure 1. The Output Ripple Voltage without the

Contribution of ESR

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